[Ifeffit] Amplitude in Artemis

[Matthew Marcus]

I don't know if this is relevant to the particular system under discussion, 
but I've had trouble with either the XANES standard or the
sample showing overabsorption or the hole effect, both of which do similar 
things to the XANES.  In EXAFS, these effects are
pretty well approximated by an overall scale, so you still get a fit, just 
with wrong weights, but in XANES, the shape changes, so
you don't get a good fit.
    mam
----- Original Message ----- 
From: <bsin1033 at mail.usyd.edu.au>
To: "XAFS Analysis using Ifeffit" <ifeffit at millenia.cars.aps.anl.gov>
Sent: Thursday, March 02, 2006 2:21 PM
Subject: RE: [Ifeffit] Amplitude in Artemis


> Dave,
>
> Thank you for your comments. By doing your way it works but one of the
> standard is not quite right, as you have also suggested. We will rerun the
> standards and try it again.
>
> Cheers,
>
> Balwant
>
> Quoting "Barton, David (DG)" <DGBarton at dow.com>:
>
>> Balwant,
>>
>> I have done a number of LCF using Athena and have found that this error
>> message is typically associated with the rejection of one of your
>> standards as a possible component in the fit.  This error message may be
>> avoided in most cases if you uncheck the options of --weights between 0
>> &1 and --Force weights to sum to 1 AND you put in an appropriate initial
>> guess for the fit.  When you rerun the fit with these options unchecked,
>> it should be obvious what caused the error since one or more of your
>> standard spectra will get a negative weight.  When this occurs it is
>> probably a good sign that the standard is not appropriate in the fit and
>> some other standard should be used.
>>
>> Hope this helps,
>> Dave
>>
>> > -----Original Message-----
>> > From: ifeffit-bounces at millenia.cars.aps.anl.gov
>> > [mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] On Behalf
>> > Of bsin1033 at mail.usyd.edu.au
>> > Sent: Thursday, March 02, 2006 3:41 PM
>> > To: XAFS Analysis using Ifeffit; Kelly, Shelly D.
>> > Cc: XAFS Analysis using Ifeffit
>> > Subject: RE: [Ifeffit] Amplitude in Artemis
>> >
>> >
>> > Hi Ifeffitters,
>> >
>> > I was trying to use the linear combination fitting function
>> > in Athena to quantifying Cr(VI) and Cr(III) species. I used
>> > two endmembers of Cr species as the two standards but the
>> > program fails and gives error message "The fit flagged a
>> > warning--probably that error bars could not be calculated.
>> > That is probably an indication that one or more of your
>> > standards are inappropriate for this fit." It appears I am
>> > either using the wrong settings or even wrong function to do
>> > the job, any suggestions. We have additional 3 spectra with
>> > variable combinations of Cr(VI) and
>> > Cr(III) as well.
>> >
>> > Cheers,
>> >
>> > Balwant Singh
>> >
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